For the first time, world governments committed themselves to write a comprehensive global agreement to reduce greenhouse gas emissions, covering developed and developing countries, to come into force in 2020.

Haven't we had climate change agreements before?

Yes, going back to 1992, but never like this. The 1997 Kyoto protocol is the world's only existing treaty stipulating cuts in greenhouse gas emissions, but the cuts only apply to developed countries and the US has never joined in. The current Kyoto targets expire next year, and only the EU among major developed countries has agreed to a continuation of them afterwards – Japan, Russia and Canada have all dropped out. Accords were also struck in 2009 and 2010 at Copenhagen and Cancun respectively, by which most countries and all of the biggest economies set out national targets on emissions curbs up to 2020. But these are voluntary, not legally binding.

Why is a legally binding treaty important?

An international treaty is far harder for future politicians to wriggle out of than commitments at a national level. They are not impossible to renege on – witness the failure of several Kyoto participants, such as Canada, to meet their targets – but they give an additional level of security which is important for investors looking to pour money into greening the economy.

Will the new agreement really be legally binding?

A last-minute compromise at Durban meant the new phase of negotiations about to start should be "a protocol, a legal instrument or an agreed outcome with legal force". The latter is the weakest option, but according to the EU it will effectively mean countries are legally bound.

Will this deal stop climate change?

No. What was discussed at Durban were the principles on which future negotiations will be based. There was no discussion of how far and how fast countries should be cutting their carbon dioxide.

Isn't that worrying?

Yes – emissions have risen by nearly 50% in the past 20 years, and they are still going up. With every year of increase, we have much less chance of keeping global temperature rises to less than 2C – beyond which scientists say climate change becomes catastrophic and irreversible.

So what happens next?

Governments will begin negotiations on what the new climate agreement should look like. According to the "Durban platform", these must wrap up in 2015 with a legal document ready to be signed. Governments will then have five years to ratify it.

What can go wrong?

Everything. This is a delicate process, as powerful national and vested interests are involved, and it has taken 20 years to get this far. Countries deciding how far and how fast to cut emissions was one of the hardest part of the process leading up to the Copenhagen summit in 2009, and the pledges declared there were inadequate to the scale of the climate challenge, according to scientists. Although the US has signed up, for instance, there is no guarantee that if a Republican president were elected next year, he or she would not rip up this agreement and refuse to negotiate.